Digitizing apparatus for digitizing a model surface using a contactless probe

ABSTRACT

A digitizing apparatus in which the surface of a model is traced by a probe which measures distance without contacting the surface, thereby forming digitizing data along the profiling path. First, drive motors for two axes are controlled simultaneously to form profiling data regarding the sectional shape of a model sectioned by a profiling plane formed by the two axes. Thereafter, the probe is moved along a second profiling direction which perpendicularly intersects a profiling direction defining an initial profiling path. Tangential vectors in the two profiling directions are obtained. Two perpendicular vectors are then decided from these tangential vectors. As a result, a normal vector at a point of intersection between the two profiling directions is computed.

BACKGROUND OF THE INVENTION

(1) Field of the Invention

This invention relates to a digitizing apparatus in which the surface ofa model is traced by a probe which measures distance contactlessly,thereby forming digitizing data along the profiling path.

(2) Description of the Related Art

In ordinary tracer control, a stylus is moved while in contact with thesurface of a model. Displacement quantities εx, εy, εz along respectiveaxes, which depend upon the tracing path on the model surface, aredetected from a tracer head. In a tracer control apparatus, thedisplacement quantities εx, εy, εz along the respective axes arecombined into a vector and the direction of a normal to the modelsurface is computed to form profiling data. When machining issubsequently performed based on the digitizing data, offset valuesconforming to the axial components of the normal vector are decided anda difference between stylus diameter and cutter diameter is corrected bya three-dimensional correction.

In digitizing based on this conventional contact profiling system, thedirection of displacement does not accurately reflect the normal linedirection of the model surface due to friction between the stylus andmodel. It is difficult to form accurate digitizing data because ofdisturbance caused by roughness of the model surface. In addition, dueto the inaccurate normal vector, the original model shape cannot bereproduced at the time of machining even if cutter diameter iscompensated for in three dimensions.

SUMMARY OF THE INVENTION

The present invention has been devised in order to solve the foregoingproblems. Its object is to provide a digitizing apparatus in which thedigitizing of a model surface is performed twice using a contactlessprobe. The direction of the normal is accurately decided to enable ahighly precise three-dimensional correction.

According to the present invention, there is provided a digitizingapparatus in which the surface of a model is traced by a probe whichmeasures distance contactlessly, thereby forming digitizing data alongthe profiling path. The digitizing apparatus comprises direction settingmeans for setting a first profiling direction which defines theprofiling path, and a second profiling direction which perpendicularlyintersects the first profiling direction, memory means for storingtangent vectors computed from an error quantity between probemeasurement distance and a reference distance in each of the profilingdirections, arithmetic means for computing, from a vertical vectorcomputed on the basis of the tangent vectors, a normal vector withregard to the surface of the model at a point of intersection betweenthe two profiling directions, and correcting means for correcting, onthe basis of the normal vector, digitizing data formed with regard tothe first profiling direction.

The digitizing apparatus of the invention is such that the surface of amodel M is digitized twice. That is, it is digitized along a firstprofiling path and along a second profiling path, using a contactlessprobe as shown in FIG. 4 so that the direction of the normal to thedigitized model can be accurately corrected. Digitizing data can beformed which includes highly precise three-dimensional correctiondirection data.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of an embodiment according to the presentinvention;

FIG. 2 is a diagram for describing contactless profiling;

FIG. 3 is a block diagram of a digitizing apparatus which controls aprobe;

FIG. 4 is a diagram of two profiling paths for digitizing a modelsurface; and

FIGS. 5 and 6 are graphs of a procedure for computing a normal vector ata point of intersection.

DESCRIPTION OF THE PREFERRED EMBODIMENT

An embodiment of the present invention will now be described in detailwith reference to the drawings.

First, contactless profiling will be described with reference to FIG. 2.This is a method in which profiling data is formed, without makingcontact with the surface of a model. A probe sensor, such as a laserprobe, measures distance contactlessly. The probe sensor generally isset to a reference distance and outputs the difference between thisdistance and an actually measured distance as an error quantity. Morespecifically, points A, B and C on the surface of a model M are taken assuitably selected sampling points. In order to form profiling data forthis surface, first a vertical distance L₁ from point A to a probe P ismeasured. Then the probe P is moved to the next sampling point B whileapplying a correcting operation which is commensurate with an errorquantity .sub.Δ L₁ (=L₁ -L_(o)) between the measured distance and areference distance L_(o) along the direction of the measurement axis.Next a vertical distance L₂ at point B is measured to compute an errorquantity .sub.Δ L₂ in the same manner. Then the probe is moved to pointC while correcting the error quantity. In this way positionalcoordinates of the three sampling points within three-dimensional spaceare decided so that profiling data along the profiling path can bedigitally sampled.

FIG. 3 is a block diagram of an example of a digitizing apparatus forcontrolling the probe P which performs the above-described contactlessprofiling.

Numeral 11 denotes a tracer control apparatus, 12Z and 12X represent DAconverters for DA-converting digital velocity signals Vx and Vz alongthe respective axes generated by the tracer control apparatus 11, 13Zand 13X designate servo circuits for the respective axes. Numerals 14Zand 14X denote motors to be driven along the Z and X axes, respectively,15Z and 15X represent position detecting pulse coders for generatingrespective pulses Pz and Px each time the corresponding motors rotatethrough a predetermined angle, 16Z and 16X denote present positionregisters for the respective axes for monitoring present position(Xn,Zn) along the axes by reverse counting, in dependence upon thedirection of movement, the pulses generated by the corresponding pulsecoders.

The tracer control apparatus 11 receives, as inputs, the necessary dataregarding commanded velocity Vcmd, profiling limits and profilingmethod, etc., from a control panel (not shown), as well as a measureddistance L from the probe P. When these inputs arrive, the tracercontrol apparatus 11 computes the error quantity .sub.Δ L between themeasured distance and the reference distance L_(o) and computes, fromthe error quantity .sub.Δ L, the velocity of the probe P in the normaldirection as well as the velocity thereof in a tangential direction.Also input to the tracer control apparatus 11 are the present axialpositions (Xn,Zn) synchronized to the computation period. The apparatus11 computes the angle of inclination of the model M and, based on thisangle of inclination and the velocities in the normal and tangentialdirections, outputs the axial velocity signals Vx and Vz to the DAconverters 12X and 12Z as profiling data. The movement of the probe P iscontrolled at the commanded profiling velocity Vcmd while the probe isheld the reference distance L_(o) above the surface of the model M.

The profiling data (Vx,Vz) output by the above-described digitizingapparatus is obtained by continuously picking up points on the profilingpath of the surface of model M. A correction corresponding to thediameter of the cutter must be performed in order to perform actualmachining based on the digitizing data. Though the cutter diametercorrection is performed in three dimensions in the direction of thenormal to the surface of model M, it is necessary, in order to performthe cutter diameter correction correctly, to calculate the direction ofthe normal to the model surface at each point on the profiling path bysome method and decide the direction of the three-dimensionalcorrection. Therefore, according to the present invention, the directionof the normal is correctly decided by performing digitizing on thesurface of the model M twice. That is, digitizing is performed along afirst profiling path 1 and a second profiling path 2, as shown in FIG.4.

FIG. 1 is a block diagram of the general construction of theabovementioned tracer control apparatus 11. Numeral 3 denotes aprofiling direction setting unit for setting profiling directions whichdefine the second profiling path 2, which forms a plurality of points ofintersection with the first profiling path 1 at predetermined intervals.Numeral 4 denotes a profiling direction computing unit for computing aprofiling direction, 5 an adder for computing the error quantity .sub.ΔL between the measured distance L and the reference distance L_(o), 6 avelocity signal generating unit for generating a normal directionvelocity signal Vn, 7 a velocity signal generating unit for generating atangential direction velocity signal, 8 a memory for storing atangential direction velocity signal Vt, 9 a normal vector computingunit for computing a normal vector at the point of intersection betweenthe two profiling paths, and 10 a profiling data generating unit forgenerating profiling data from the computed profiling direction andvelocity signals.

Contactless profiling with the digitizing apparatus having theconstruction designed above will now be described.

First, the probe P is moved along the first profiling path 1, theresults are digitized and a tangential vector, which is based on theerror quantity between measured distance and reference distance of theprobe P computed at this time, is stored in the memory 8. Next, theprobe P is moved along the path 2 formed by the profiling directionsetting unit 3. In this case, the profiling direction setting unit 3,which sets the second profiling direction so as to form a plurality ofpoints of intersection with the first profiling path 1 at apredetermined interval, defines the profiling path 2 in synchronizationwith the computation period of the profiling direction computing unit 4,thereby forming the plurality of points of intersection. The reason forthis is convenience in terms of the computations when performing aprofiling data correction, described below.

FIG. 5 is a portion of the second path along which the probe P is movedat right angles to the first path 1. Let (Xp,Yp,Zp) and (Xq,Yq,Zq)respectively represent machine positions at points P and Q astride thepath 1, and let 1₁ and 1₂ respectively represent tangential vectorbeginning and end points on either side of the point of intersectionwith the path 1. A perpendicular vector which perpendicularly intersectsthe tangential vector of the profiling path 1, and a perpendicularvector which perpendicularly intersects a vector PQ obtained by linearlyapproximating the profiling path 2, are computed as follows:

Based on the length l₁ =√(X2-X1)² +(Y2-Y1)² of the projection of thetangential vector l shown in FIG. 6 onto the x-y plane, the formerperpendicular vector is a vector perpendicular to this tangential vectorl (l₁,Z2-Z1). That is, this perpendicular vector is expressed as followsin the original x-y-z space: ##EQU1##

Similarly, based on the length l₂ =√(Xp-Xq)² +(Yp-Yp)² of the projectionof PQ onto the x-y plane. The vector which perpendicularly intersectsthe vector PQ is a vector perpendicular to the vector PQ (l₂,Zq-Zp).That is, this perpendicular vector is expressed as follows in theoriginal x-y-z space: ##EQU2## The normal vector computing unit 9computes the two perpendicular vectors from the tangential vector storedin the memory 8, adds these two vectors and outputs the sum asdigitizing data in the normal direction with regard to the modelsurface.

It is of course permissible to store a tangential vector at each pointof the profiling path 1 in the memory 8 from the beginning based on thedata from the computing unit 4. Further, depending upon the setting ofthe second profiling path 2, the method of computation between the twoperpendicular vectors performed by the computing unit 9 can be changed.For example, the normal vector can be computed by a weighted mean whichis appropriately weighted. In any case, by thus accurately deciding thenormal direction with respect to digitizing data formed along theinitial profiling path 1, an accurate three-dimensional correction canbe performed when performing machining by the digitizing data.

Though an embodiment of the present invention has been described for acase where the tracer control apparatus 11 is expressed as beingcomposed of the hardware shown in FIG. 1, the invention is not limitedthereto but can be modified in a variety of ways without departing fromthe scope of the claims.

The digitizing apparatus of the present invention is such that thedigitizing of a model surface is performed twice using a contactlessprobe. The direction of the normal to the digitized model can beaccurately decided, and digitizing data which includes highly precisethree-dimensional correction direction data can be formed.

I claim:
 1. A digitizing apparatus in which the surface of a model is traced by a probe which measures distance contactlessly, thereby forming digitizing data along the profiling path, said digitizing apparatus comprising:profiling direction setting means for setting a first profiling direction for defining the profiling path, and for setting a second profiling direction which perpendicularly intersects the first profiling direction; memory means, operatively connected to said profiling direction setting means, for storing tangent vectors computed from an error quantity between a probe measurement distance and a reference distance in said first and second profiling directions; arithmetic means, operatively connected to said memory means, for computing from a perpendicular vector computed on the basis of said tangent vectors, a normal vector with regard to the surface of the model at a point of intersection between said first and second profiling directions; and correcting means, operatively connected to said arithmetic means, for correcting, on the basis of the normal vector, digitizing data formed in accordance with said first profiling direction.
 2. A digitizing apparatus according to claim 1, wherein said profiling direction setting means sets said second profiling direction so as to form a plurality of points of intersection with said first profiling path at predetermined intervals.
 3. A digitizing apparatus according to claim 1, wherein said memory means stores a tangential vector as a functional value of the error quantity.
 4. A digitizing apparatus according to claim 1, wherein said arithmetic means computes a normal vector at a point of intersection by adding two perpendicular vectors. 